What exactly is quantum entanglement?

Imagine you have two coins, each with a different front or back, you hold one and I hold one, and we are very far away from each other. Pen "Fun" Pavilion www.biquge.info We toss them in the air, catch them, and slap them on the table. When we look at the results with our hands open, we expect each to see the "head" 50% of the time, and each of them to get the "tail" 50% of the time. In the normal non-entangled universe, your results and mine are completely independent of each other: if you get a "heads" result, the probability that my coin will show up as "heads" or "tails" is still 50% each, but in some cases, the results will be entangled with each other, that is, if we do this experiment and you get "heads", then without me telling you, you will be 100% sure that my coin will show "tails", Even if we are light-years apart and not even a second has passed.

In quantum physics, we are usually entangled not with coins but with individual particles, such as electrons or photons. For example, if each photon spins +1 or -1, if two photons are entangled with each other, you measure the spin of one of them and instantly know the spin of the other, even if it spans half the universe. Until you measure the spin of any particle, they exist in an indeterminate state; But once you measure one of them, both are immediately known. We've done an experiment on Earth where we separated two entangled photons many kilometers apart and measured their spins at nanosecond intervals. We found that if the measurement found that one of them had a spin of +1, we knew that the other was -1 at least 10,000 times faster than communicating at the speed of light.

After creating two entangled photons, even if they are far apart, we can learn about the other by measuring the state of one of them.

Now back to the question: can we use this property of quantum entanglement to communicate with distant star systems? The answer is yes, if you consider that measuring from a distant place is also a form of "communication". However, when we talk about "communication" in general, we usually want to know what you are trying to do. For example, you can keep an entangled particle in an uncertain state, board a spacecraft heading to the nearest star, and then command the spacecraft to look for rocky planets in the habitable zone of that star. If found, a measurement is made to put the particle in the +1 state, and if it is not found, a measurement is made to put the particle in the -1 state.

Therefore, you speculate that when the spacecraft makes measurements, if the particles left on Earth appear to be in the -1 state, you know that the spacecraft has discovered a rocky planet in the habitable zone; Particles left on Earth will appear in a +1 state, indicating that the spaceship has not yet discovered the planet. If you know that the spacecraft has already taken measurements, you should be able to measure the particles left on Earth yourself and immediately know the state of another particle, even if it is many light-years away.

It's a clever plan, but there's a catch: only you ask a particle "What state are you in?" "[i.e., measurement] only works, but if you measure an entangled particle and force it to be a particular state, you break the entanglement, and the measurement you make on Earth is completely irrelevant to the measurement you make next to a distant star. If you take a measurement at a distance and have a particle state of +1, the result will of course be -1 on Earth, telling you about particles that are light-years away. But you can't measure without breaking the entanglement, and if the entanglement does, it means that no matter what the outcome, the probability that you have a particle on Earth that is +1 or -1 is 50%, and it has nothing to do with a particle a few light-years away.

For example, my friend and I are on the horizon, but each holding a quantum coin in our hands, they must be positive and negative. I can tell the status of the other party's coin by the coin in my hand. But I can't change the coin in my friend's hand by changing the coin in my hand (it is okay to change it, but the result is random. Just like the three-body problem said to play billiards, the billiards that is hit is flying out in any direction, only obeying probability, not obeying the laws of physics). Now quantum communication seems to be another thing, as if a law has been discovered, playing two billiards at the same time, although the direction of the two billiards is arbitrary, but the center of the angle between the two billiards balls is the direction of hitting the ball. Then establish two links, one of which is a normal link, to inform the other party of the direction of the other ball. In this way, the real receiver can get useful information (the angle of the ball) through the direction of the billiard ball in the quantum state and the direction of another billiard ball passing through. The eavesdropper can't get the quantum state, so it can't eavesdrop. It's a bit wordy, but let's go back to the coin. My friend and I each have a magic coin (A and A\', and they always keep one positive and the other negative. I want to control the heads and tails of the coin and pass the message to my friends; But I couldn't, no matter how careful I put the coin on the table, the coin insisted on its randomness, and the heads and tails appeared indefinitely. Naturally, my friend had no way of knowing what I was sending to him. Later, I found a magic coin (B) with new characteristics. Even if I throw them together (A and B) upwards, they must be the same; Throw them downwards and they must be different. In this way, I told my friend by phone that after each flip of a coin, the state of Coin B. My friend knows how I toss a coin every time. Although the speed of communication is still the speed of making a phone call, it is absolutely confidential. (To be continued.) )